chore: add Dependabot dependency update automation

## Summary

Add Dependabot configuration so `bdk-dart` can automatically surface dependency update PRs for the package, demo app, Rust native crate, and GitHub Actions.

This should help maintainers notice outdated dependencies earlier without manually checking each ecosystem.

## Scope

- Add `.github/dependabot.yml`.
- Configure updates for root Dart/pub dependencies.
- Configure updates for `bdk_demo` Dart/Flutter dependencies.
- Configure updates for Rust/Cargo dependencies under `native`.
- Configure updates for GitHub Actions used by repository workflows.
- Use a low-noise schedule, such as weekly updates.
- Group related updates where it keeps review manageable.

## Out of scope

- Updating dependency versions in this PR.
- Changing lockfiles.
- Changing CI workflow behavior.
- Changing native build or release logic.
- Enabling auto-merge.

## Acceptance criteria

- [ ] Dependabot is configured for pub, Cargo, and GitHub Actions ecosystems.
- [ ] The configuration covers the repository root, `bdk_demo`, `native`, and `.github/workflows`.
- [ ] The update schedule is intentionally low noise.
- [ ] The PR only adds Dependabot configuration unless maintainers request otherwise.
- [ ] The configuration is valid YAML and follows GitHub Dependabot conventions.